Circuit arrangements for the isolated sensing of binary electric signals of different pulse heights at a transmission line are known, for example, from EP 0 398 456 B1 or from a prospectus of Hewlett Packard entitled "High-Linearity Analog Optocouplers", 1993, pages 1 to 16.
In networks with central powering, both the supply voltage and the supply current can generally be measured at the terminal station. This was done formerly using simple analog pointer-type instruments which were tied to the supply potential.
Since the introduction of operator terminals, however, it must be possible to process the two measured values digitally. This involves difficulties with electrical isolation, because the potentials of the voltages on the transmission lines may be up to 2 kV above ground potential, with voltage and current values occurring between the go conductor and the return conductor in the ranges of 0 V to approximately 300 V and 0 A to approximately 70 mA, respectively.
One possibility of making an isolated measurement is to use an analog-to-digital converter (ADC) which is tied to the potential of the transmission line and whose output can be taken at a potential near ground potential with the aid of an optocoupler interface. This, however, requires a separate power supply which is tied to the potential of the transmission line. The supply current from the transmission line cannot be used because in the event of an interruption no current flows there and only a very small current may be consumed in a path parallel to the line. Another problem is that any fairly linear, temperature-independent measurement using optocouplers can only be implemented with complex compensation circuits. In addition, if active components, such as transistors, which necessarily form part of an ADC circuit, are used at an elevated potential, the so-called latch-up problem arises, which, if the supply voltages rise above or fall below the rated values even by small amounts, may result in thermal destruction of the components.
CH 676 393 A5 discloses a circuit arrangement for converting an analog measurement voltage into an isolated digital signal in which the measured voltage is coupled out near ground potential via an optoelectronic coupling element consisting of a light-emitting diode (LED) and a phototransistor tied to the potential of the transmission line. The voltage measurement itself is made by means of a capacitor and a circuit which controls the charging and discharge of the capacitor. Such a capacitor, however, is necessarily quite voluminous. In addition, its discharge curve follows an exponential function, which impairs the accuracy of the measurement.
The above-cited EP 0 398 456 B1 discloses a circuit arrangement for the isolated sensing of binary electric signals of different pulse heights which includes an optocoupler that is connected in series with an electronic circuit module to the go and return conductors of the circuit arrangement. This circuit arrangement can be operated without clock pulses but has the disadvantage of being unsuitable for measuring analog signals because of the optocoupler's high degree of nonlinearity. It also requires a high-potential constant-current source for the optocoupler, which causes the above-mentioned latch-up problem.
The above-cited prospectus of Hewlett Packard also describes circuit arrangements with pairs of optocouplers in which the analog measured values are obtained near ground potential by comparing the two signals provided by the two optocouplers, since the individual optocouplers themselves deliver only analog signals. This, however, presents new problems because of the different aging rates of the two optocouplers.